How "sharp" should HSS lathe tools be?

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zippyslug31

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I'm dealing with pretty poor surface finishes and am fighting this on several fronts: lack of overall experience, initial machine setup, possible machine problems, etc. On some advice, I didn't add to my learning curve by also trying to learn grinding geometry right away so I picked up a "ready to go" HSS set similar to this.

I've since learned that one of my problems is the tool I'm trying to use doesn't have any nose radius so I added a small radius which helped a little. One would think that these would work right from the supplier, but this was foolishly optimistic apparently.

I've been reading a ton of posts here and there's a lot of confusing information of exactly how sharp the edges are supposed to be when doing the final honing of the cutting edge. Some say "razor sharp", yet others say it's a relative term and it's not even "knife sharp". I'd say that all of the tooling that came in my set have "crisp corners", but you would be hard pressed to slice skin.

Can anybody help to clear this one topic for me, or maybe this is one of those topics where everybody has a varying opinion? :boxing:
Thanks in advance!
 
I hone mine with the blue “extra fine” EZE Lap diamond file. I like the control these give and they also remove material fast. More importantly perhaps is to make sure that there is no curve or unwanted bevel rolled over on the edge when you hone, — that pretty much defeats your grinding geometry and nullifies any benefit you get from using a fine stone or stick.
 
I would start by just grinding the tool with a flat top, a small relief angle on the 2 cutting edges and turn some aluminum 500 RPM, 0.1 mm depth of cut and a slow feedrate 0.05 mm/rev. The tool should cut fine with a poor surface finish.
Than grind a small nose radius on the tip. Just by slightly rotating the tip against the grinding wheel with very very very little pressure once.
Do another turning pass, the surface finish should be better. If so, you could grind the nose radius larger for a better surface finish. Just repeat the above step once and try again. That will give you an idea about the size of the nose radius. You don't need more than 0.5 mm nose radius.
Once turning aluminum is going OK, you could try brass and after that some mild steel. When you start turning steel, you should start worrying about feed and speed settings.
I don't bother to grind a chip breaker or lap the cutting edges and use a €10,-- "diamond disk" for grinding. The tools perform well and are easy to (re)grind.
Lapping the tools could make them really good or ruin the cutting edges.
I use these kind of holders to (re)grind my HSS tools

S7300531.jpg
 
I hone mine until... wait, I don't hone mine..

If you're fighting finish problems, honing is not the issue. Check setup and general geometry. You should be able to get an excellent finish with tools ground on even a course wheel. Honing is for very light and/or precise setups. I suggest ignoring such small details until you've got a handle on the big stuff. "10* everywhere" is what I suggest for those getting started - it's not the best geometry for anything, but it will cut about anything well. Pics of your cutter and setup may help (take a pic of your setup before breaking it to take pics of your cutter). Typically these sorts of problems are something fundamental - something missed that, once corrected, will never bother you again.

GsT
 
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